1. Field of the invention
The present invention relates to a lithium rechargeable battery. More particularly, the present invention relates to a lithium rechargeable battery, which includes a separator having excellent mechanical strength, swelling resistance and heat resistance.
2. Description of the Prior Art
Recently, as the portable electronic instruments have been designed to have a low weight and a compact size, a battery used as a drive source for such instruments have been required to have a low weight and a high capacity. Particularly, active and intensive research and development for a lithium rechargeable battery has been conducted, because a lithium rechargeable battery has a drive voltage of 3.6V or higher, which is higher than the drive voltage of a Ni—Cd battery or Ni-MH battery, widely used as a power source for portable electronic instruments, by at least thee times, and provides a higher energy density per unit weight
A lithium rechargeable battery generates electric energy by redox reactions occurring upon the lithium ion intercalation/deintercalation in a cathode and an anode. A lithium rechargeable battery is obtained by using a material capable of reversible lithium ion intercalation/deintercalation as a cathode active material and an anode active material and introducing an organic electrolyte or polymer electrolyte between the cathode and the anode.
In general, a lithium rechargeable battery includes an electrode assembly which includes an anode plate, a cathode plate and a separator interposed between both electrode plates and which is wound into a predetermined shape such as a jelly-roll shape, a can for housing the electrode assembly and an electrolyte, and a cap assembly mounted to the top of the can. The cathode plate of the electrode assembly is electrically connected to the cap assembly via a cathode lead, while the anode plate of the electrode assembly is electrically connected to the can via an anode lead.
The separator has its basic function of separating the cathode and the anode from each other so as to prevent a short circuit in a lithium rechargeable battery. Additionally, it is important for the separator to suck an electrolyte necessary for carrying out electrochemical reactions in the battery and to maintain a high ion conductivity. Particularly, in the case of a lithium rechargeable battery, the separator is required to have an additional function to prevent a substance capable of inhibiting the electrochemical reactions from moving within the battery or to ensure the safety of the battery under the abnormal conditions. Generally, the separator includes a microporous polymer membrane based on polyolefin such as polypropylene or polyethylene, or a multilayer membrane including multiple sheets of such membranes. Such conventional separators have a sheet-like or film-like porous membrane layer.
The sheet-like separator has a disadvantage in that the sheet-like separator may shrink while the pores of the porous membrane are blocked, when heat emission occurs by an internal short circuit or an overcharge condition Therefore, if the sheet-like separator is shrink by such internal heat emission of the battery, there is a portion that is not covered by the separator and thus allows the cathode and the anode to be in direct contact with each other, resulting in ignition and explosion of the battery.
The film-like separators ensure the safety of a battery upon heat emission caused by a short circuit via a so-called shutdown action for interrupting lithium ion movement (i.e., current flow) by blocking the pores with a softened polypropylene or polyethylene resin. However, the separators are still disadvantageous when an internal short circuit occurs. For example, in the nail test (penetration), as a substitutive test simulating an internal short circuit condition, heat emission temperature may locally reach several hundred degrees ° C. depending on the test conditions, and thus the porous membrane layer is deformed by the softening or loss of the resin. In addition to this, the test nail penetrates through a cathode and an anode, thereby causing an abnormal overheating phenomenon. Therefore, such means for utilizing the shutdown effect of a resin cannot be an absolute safety means against an internal short circuit.
Additionally, lithium dendrite is formed totally on a film-like separator upon overcharge. This is because there is a gap between an anode and the film-like separator, and thus lithium ions that cannot infiltrate into the anode are accumulated in the gap between the anode and the film, resulting in precipitation of lithium metal. If lithium precipitation occurs over the whole surface of the film, such lithium dendrite penetrates through the film-like separator so that a cathode may be in direct contact with an anode. At the same time, side reactions may occur between lithium metal and an electrolyte to cause heat emission and gas generation, resulting in the ignition and explosion of a battery.
Moreover, such a film-like separator may generate a harder short circuit, because in the event that a polyolefin-based film separator has a portion damaged by the initial heat emission, additional portions adjacent to the damaged portion may be shrink or molten continuously, thereby increasing the area lost by the combustion of the film separator.